The consumed power during cutting depends on whether the cutting tool is brand-new or secondhand and on the way in which it is mounted on the machine. Even with the same tool, the consumed power is greatly influenced by the flow of chips and foreign matter adhering to the cutter edge.
In controlling cutting tools in which the consumed power fluctuates greatly between the individual cutting tools and with the cutting conditions, it is necessary, instead of detecting momentary fluctuations in power, to detect the amount of increase in the consumed power due to wear or breakage with high accuracy and to control the tool based on the detected values.
Heretofore, a controller was proposed in Examined Japanese Patent Publication 57-57223 in which the depth of cut by a grinder is controlled on the basis of the fluctuations in the power consumed by a motor.
In this controller, no-load power of the motor is extracted from the noise-removed consumed power and is compared with damped loaded power. A control signal is given to the grinder when the loaded power exceeds the no-load power.
But this type of controller has a problem in that since the loaded power to be detected is a momentary value during cutting, if this controller is used for the control of such a cutting machine as mentioned above, malfunction may result due to power fluctuation during cutting.
For example, if the damping factor of the detected value is increased to keep the sampling value to be compared and the consumed power from getting too close to each other and thus to prevent malfunction resulting from difference in power among individual cutting tools due to the fact that some are brand-new and some are secondhand, the response time may increase unduly, thus worsening followability. In contrast, if the damping factor is set at such a value that the difference between the sampling value and the consumed power is small in order to improve followability, the controller will malfunction if the consumed power increases even momentarily by the influence of chips or foreign matter adhering to the cutter edge. This makes it impossible to give tool exchange instructions stably.
Also, in the above device, no monitoring period is specified during which the no-loaded power and the power during cutting are compared with each other but monitoring has to be kept on all the time. Thus, if the no-load power changes due to changes in the revolving speed or the direction of rotation of the motor while in operation, the controller may malfunction due to these changes.